Optocoupler transistor interface
Worst-case CTR, LED resistor and pull-up range with a first-order capacitive rise time.
Development preview. Check the assumptions below before relying on a result. Calculations run in your browser.
How it calculates
Assumptions
- Aging and temperature factors multiply minimum CTR. LED current regulation and VF tolerances are not modeled.
- Rise time is only the pull-up/capacitance term; optocoupler storage, propagation and phototransistor turnoff delay are additional.
Worked example
These are the calculator's default inputs. Change any input above to recalculate.
Inputs
- LED drive voltage
- 5 V
- LED forward voltage
- 1.2 V
- LED current
- 0.005 A
- Minimum CTR
- 50 %
- CTR retained after aging
- 0.8
- CTR temperature factor
- 0.8
- Output supply
- 3.3 V
- Maximum logic-low voltage
- 0.8 V
- Optocoupler saturation voltage
- 0.2 V
- Chosen pull-up
- 10000 Ω
- Total output capacitance
- 5e-11 F
- Maximum 10–90% rise time
- 0.00001 s
Results
- LED series resistor
- 760 Ω
- Minimum pull-up for logic low
- 1562.5 Ω
- Maximum pull-up from rise time
- 91023.9 Ω
- LED resistor power
- 0.019 W
- CTR-limited collector current
- 0.0016 A
- Estimated low voltage
- 0.2 V
- RC 10–90% rise time
- 0.00000109861 s